1. Field of the Invention
The present invention relates to a reaction force control apparatus for controlling reaction force to be applied to an operation unit in a steering system of a vehicle.
Priority is claimed on Japanese Patent Applications No. 2004-193011 and No. 2004-193013, filed Jun. 30, 2004, the contents of which are incorporated herein by reference.
2. Description of the Related Art
An electrical power steering apparatus is known as a steering apparatus for a vehicle. In an electrical power steering apparatus, a steering shaft connected to a steering wheel and a turning mechanism for turning steerable wheels are mechanically connected, and an electrical motor for assisting steering force is associated with the turning mechanism. In general, a driving torque command (drive current) for the electrical motor is controlled so that an assisting steering force is increased as the steering operation torque applied to the steering shaft increases.
Moreover, as such an electrical power steering apparatus, a steering system is known in which, in order to restrain irregular movement of the vehicle due to disturbance, a vehicle movement (e.g., a yaw rate) is measured by a measuring means, a drive torque correction value for compensating for the vehicle movement is calculated, a drive torque command is calculated by subtracting the drive torque correction value from a drive torque base command that is set depending on the steering operation torque, and the electrical motor is controlled according to the drive torque command (see, for example, Published Japanese Patent No. 3229074). In the steering system constructed as explained above, even during, for example, a normal turning state in which a yaw rate appears, the drive torque correction value is generated so as to reduce the yaw rate, i.e., so as to control the vehicle toward a straightforward state. Accordingly, the drive torque correction value may be defined as a reaction force with respect to an assisting force.
In general in a related art, when the reaction force is controlled depending on yaw rate information, a control operation is performed so that the reaction force (the drive torque correction value) is increased as the yaw rate increases. In other words, the driving torque of the electrical motor is controlled so that the assisting steering force by the electrical motor is decreased as the yaw rate increases. By such a control operation, a steering state is stabilized when the yaw rate is large.
However, when the reaction force is controlled in the aforementioned manner, the reaction force is controlled to be great due to a high yaw rate during a driving state in which steering operation torque is large, e.g., during a turning state with a high lateral acceleration. As a result, a large manual steering operation force is required (in other words, the steering wheel is felt heavy), and thus a problem is encountered in that steering operation feel is degraded.
Moreover, the movement of a vehicle has a phase lag with respect to a steering operation, e.g., a yaw rate has a phase lag with respect to an input to a steering wheel. When the frequency of an input to a steering wheel is higher than the yaw rate resonance frequency, the phase of the yaw rate is reversed with respect to the steering speed. In general, a greater reaction force should be generated as the yaw rate increases; however, during such a state, because the reaction force is generated with a reversed phase with respect to the steering operation, the steering wheel can be operated with a small steering operation force (i.e., the operation of the steering wheel is felt light), and a so-called “relaxed steering operation feel” is experienced. As a result, steering feel is degraded, and the object of reaction force control cannot be achieved.